East Staffs Gull
Well-known member
Could always multiply 12345679 by 9.
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[Finance] Maths Tips and Tricks
- Thread starter Brightonfan1983
- Start date
clapham_gull
Legacy Fan
- Aug 20, 2003
- 25,446
I'm a huge fan of "spell checking" algorithms.
Ever had to match up two lists of things which contain "matches" but humans have spelt / expressed them in a different way ?
Jaccard Coefficient is your friend. I also like the vector space model and voted remain,
Ever had to match up two lists of things which contain "matches" but humans have spelt / expressed them in a different way ?
Jaccard Coefficient is your friend. I also like the vector space model and voted remain,
KingKev
Well-known member
The sum of the numbers 1 to n is (n x (n+1))/2
Eg you want to add up 1+2+3+....+10 the answer is
10x11 = 110 and divide by 2 = 55
And obviously can use the same formula for any run of numbers y to n by expanding to ((n x (n+1)) - (y x (y+1)) / 2.
the sum of the numbers 11-20 is therefore
(20x21) = 420 less 110 [ie (10x11)] = 310 and divide by two = 155
...Or you could just know that the sum of 11-20 has to be exactly 10x10 (100) more than the sum of 1-10, but that's less fun
Eg you want to add up 1+2+3+....+10 the answer is
10x11 = 110 and divide by 2 = 55
And obviously can use the same formula for any run of numbers y to n by expanding to ((n x (n+1)) - (y x (y+1)) / 2.
the sum of the numbers 11-20 is therefore
(20x21) = 420 less 110 [ie (10x11)] = 310 and divide by two = 155
...Or you could just know that the sum of 11-20 has to be exactly 10x10 (100) more than the sum of 1-10, but that's less fun
Is it PotG?
Thrifty non-licker
How does an android phone calculator compare? Slower, different answers?
OzMike
Well-known member
Multiplying by 11.
If you have a 2 digit number who’s sum total is less than 10 then you just add the two numbers together and put the resulting number between the original ones.
Eg. 23 x 11 = 253
63 x 11 = 693
If you have a 2 digit number who’s sum total is less than 10 then you just add the two numbers together and put the resulting number between the original ones.
Eg. 23 x 11 = 253
63 x 11 = 693
That leads me into my one that multiples of 9 always add up to 9 except for some exceptions like 99 where they add to a multiple of 9. I also always multiply by 10 then halve it for 5x. My primary school maths teacher taught us tricks like this though I think I have forgotten more than I remember. Its why I fell in love with maths and then computing.
9 is a magic number the multiplication singular numbers add up to 9’s
1x9=9
2x9=18. 1+8=9
3x9=27. 2+7=9
4x9=36. 3+6=9
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25x9=225. 2+2+5=9
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Or in this case 2 x is the magic number (see #14)
father_and_son
Well-known member
If all the digits of a number add up to something that is divisible by 3, then that number is divisible by 3
Only ever found that useful in the numbers round on Countdown
Likewise, if the last two digits divide by 4, the whole thing does... Handy for checking for leap years and Olympics.
Nottseagull
Well-known member
Numbers are infinite, so I think you've got that the wrong way round!
Just the same but for a fraction of the cost.
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It might just be down to luck. I went to a poor Primary School where they took on a lot of new ideas for teaching. One of those was to not teach the times tables or long division or any of these tricks. I got lucky when a stand in deputy head arrived and saw the poor level of maths in our class and taught us all of this. He also got everyone in the school playing chess but thats another subject. It was like a revelation to me. Its not like the new methods at the time were bad but its great to have more than one way of doing it and some work better for some and vice versa.
Nottseagull
Well-known member
The repeat of this program has just started on Radio 4 but I don't know when the item will be. Did you know that there are no flights between Norwich International and London?
Nottseagull
Well-known member
That's a bit like saying that if the last two digits are divisible by 10, then the whole number is.... which is kind of obvious. Also, doesn't quite work for all leap years - 1900 was an Olympic year but not a leap year.
symyjym
Banned
The true formula for leap years is divisible by 4 unless its evenly divisible by 100 it isnt...unless its evenly divisible by 400 then it is.